There are several bootable Linux distributions that come prepackaged with the correct drivers and software necessary for wireless and wired network sniffing. All of these include Kismet and Ethereal or Wireshark. Below are some that are available and free:

Backtrack
Backtrack is the result of two highly respected bootable penetration toolsets combining their efforts toward one unified bootable CDROM. For additional information, go to www.remoteexploit.org

Professional Hacker’s Linux Assault Kit (Phlack)
Includes many security tools and wireless auditing and scanning software. For additional information, go to www.phlak.org.

Knoppix Security Tools Distribution (Knoppix-STD)
A general-purpose collection of security tools on a bootable Linux image. For additional information, go to www.s-t-d.org.